The exhaust from internal combustion engines contains hydrocarbons, carbon monoxide and nitrogen oxides which must be removed to levels established by various government regulations. The aforementioned three-way catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.
Typical three-way catalysts which exhibit good catalytic activity and long life contain one or more platinum group metals (e.g. Pt, Pd, Rh, Ru and Ir) located upon a high surface area porous refractory oxide support, e.g. a high surface area alumina coating. The porous refractory oxide support is carried on a suitable non-porous refractory substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure or refractory particles such as spheres, pellets or short extruded segments of a suitable refractory material.
Three-way catalysts are currently formulated with complex washcoat compositions containing stabilized alumina, an oxygen storage component (primarily stabilized ceria) and precious metal catalytic components. The term "oxygen storage component" is used to designate a material which is capable of being oxidized during oxygen-rich (lean) cycles of engine operation and reduced during oxygen-poor (rich) cycles of engine operation.
The art has devoted a great deal of effort in attempts to improve the efficiency of palladium containing three-way catalysts. Thus, in an article in Third Int. Cong. Catal. and Auto Poll. Controls, Pre-print Vol. 1, pages 125 to 135, the authors, Dettling et al., describe the inclusion of a low temperature catalyst component (Pd/Al.sub.2 O.sub.3) and a high temperature catalyst component (Pd/CeO.sub.2) in the same catalyst composition for high activity under both low and high operating temperatures.
Additionally, U.S. Pat. Nos. 5,057,483 and 4,727,052 and EPA 92302928.4 describe palladium containing three-way catalysts.
Catalysts of this type exhibit greatly improved three-way catalytic activity over the prior art, even after extended high temperature aging. Three-way catalysts have the desirable feature of converting pollutants found in automotive exhaust to harmless gases. For instance, hydrocarbons, carbon monoxide, and nitrogen oxides are converted to carbon dioxide, water, and nitrogen. However, these and other similar three-way catalysts also have the undesirable feature of converting sulfur compounds found in automotive exhaust, such as sulfur dioxide, to hydrogen sulfide under specific vehicle operating conditions. Also, in many cases, as the three-way catalyst activity increases through formulary improvements, the hydrogen sulfide generation activity usually also increases. Additives, such as NiO, can be included in the three-way washcoat to reduce its hydrogen sulfide generation activity, however, these additives can also reduce the thermal durability of the three-way catalyst, especially at extended high temperature aging. The mechanism of this deactivation is not completely clear. However, it is thought to result from undesirable solid state reactions which occur between the catalyst components and the hydrogen sulfide suppressing additive when the composite catalyst is exposed to high temperatures.